Humidity sensing by optically interfacing with spectrally absorptive materials
The measurement of humidity is rapidly increasing in importance in physical, chemical and biological processes as industry attempts to improve quality and production rates by better control of relative humidity (RH). A novel optical fibre humidity sensor suitable for use in an optical fibre multi-point distributed sensing system has been developed, thus for the first time allowing multipoint measurements of humidity along an optical fibre which can be distributed within an industrial plant or machine without the need for electrical power supplies. The novel point sensor is based on the principle of using the absorption spectrum of a colorimetric reagent (cobalt chloride) immobilised on the surface of the core of a multimode optical fibre by employing a thin gelatin film. The single point sensor has an insertion loss of less than 0.2 dB. Two-wavelength detection is employed to provide referencing to eliminate common mode intensity variation. The basic design, construction and testing of an experimental sensor in the humidity region 20 to 80% RH and temperature range 25 to 50 °C is described. Resolution and Repeatability can be better than 2% RH with a time constant of 0.5 second. A two-wavelength optical time domain reflectometer (OTDR) has been developed and a four-sensor network has been built. The resolution of measurement on the network can be better than 4% RH, but because of noise in the OTDR the resolution value is dependent on averaging time in the instrument.